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Single-lined Spectroscopic Binary Star Candidates in the RAVE Survey
Repeated spectroscopic observations of stars in the RAdial VelocityExperiment (RAVE) database are used to identify and examine single-linedbinary (SB1) candidates. The RAVE latest internal database (VDR3)includes radial velocities, atmospheric parameters, and other parametersfor approximately a quarter of a million different stars with slightlyless than 300,000 observations. In the sample of ~20,000 stars observedmore than once, 1333 stars with variable radial velocities wereidentified. Most of them are believed to be SB1 candidates. The fractionof SB1 candidates among stars with several observations is between 10%and 15% which is the lower limit for binarity among RAVE stars. Due tothe distribution of time spans between the re-observation that is biasedtoward relatively short timescales (days to weeks), the periods of theidentified SB1 candidates are most likely in the same range. Because ofthe RAVE's narrow magnitude range most of the dwarf candidates belong tothe thin Galactic disk while the giants are part of the thick disk withdistances extending to up to a few kpc. The comparison of the list ofSB1 candidates to the VSX catalog of variable stars yielded severalpulsating variables among the giant population with radial velocityvariations of up to few tens of km s-1. There are 26 matchesbetween the catalog of spectroscopic binary orbits (S_{B^9}) and thewhole RAVE sample for which the given periastron time and the time ofRAVE observation were close enough to yield a reliable comparison. RAVEmeasurements of radial velocities of known spectroscopic binaries areconsistent with their published radial velocity curves.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Pre-main sequence spectroscopic binaries suitable for VLTI observations
Context: A severe problem for research in star-formation is that themasses of young stars are almost always estimated from evolutionarytracks alone. Since the tracks published by different groups differ, itis often only possible to give a rough estimate of the masses of youngstars. It is thus crucial to test and calibrate the tracks. Up to now,only a few tests of the tracks could be carried out. However, it is nowpossible with the VLTI to set constrains on the tracks by determiningthe masses of many young binary stars precisely. Aims: In order to usethe VLTI efficiently, a first step is to find suitable targets, which isthe purpose of this work. Given the distance of nearby star-formingregions, suitable VLTI targets are binaries with orbital periods betweenat least 50 days and a few years. Although a number of surveys fordetecting spectroscopic binaries have been carried out, most of thebinaries found so far have periods that are too short. Methods: We thussurveyed the Chamaeleon, Corona Australis, Lupus, Sco-Cen, and ρOphiuci star-forming regions in order to search for spectroscopicbinaries with periods longer than 50 days, which are suitable for theVLTI observations. Results: As a result of the 8 year campaign, wediscovered 8 binaries with orbital periods longer than 50 days. Amongstthe newly discovered long-period binaries is CS Cha, which is one of thefew classical T Tauri stars with a circumbinary disk. The survey islimited to objects with masses higher than 0.1 to 0.2 M_ȯ forperiods between 1 and 8 years. Conclusions: We find that the frequencyof binaries with orbital periods ≤3000 days is of 20±5%. Thefrequency of long and short period pre-main sequence spectroscopicbinaries is about the same as for stars in the solar neighbourhood. Intotal 14 young binaries are now known that are suitable for massdetermination with the VLTI.based on observations obtained at the European Southern Observatory atLa Silla, Chile in program 62.I-0418(A); 63.I-0096(A); 64.I-0294(A);65.I-0012(A); 67.C-0155(A); 68.C-0292(A); 68.C-0561(A); 69.C-0207(A);70.C-0163(A); 073.C-0355(A); 074.A-9018(A); 075.C-0399(A-F). Tables 2,3, 5, 6, 8, 10, 12-20 are only available in electronic form athttp://www.aanda.org
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| On the age of the TW Hydrae association and 2M1207334-393254
Aims.We have estimated the age of the young moving group TW HydraeAssociation, a cohort of a few dozen stars and brown dwarfs located nearthe Sun which share the same kinematic properties and, presumably, thesame origin and age. Methods: .The chronology has been determinedby analyzing different properties (magnitudes, colors, activity,lithium) of its members and comparing them with several well-known starforming regions and open clusters, as well as theoretical models. Inaddition, by using medium-resolution optical spectra of two M8 membersof the association (2M1139 and 2M1207 - an accreting brown dwarf with aplanetary mass companion), we have derived spectral types and measuredHα and lithium equivalent widths. We have also estimated theireffective temperature and gravity, which were used to produce anindependent age estimation for these two brown dwarfs. We have alsocollected spectra of 2M1315, a candidate member with a L5 spectral typeand measured its Hα equivalent width. Results: .Our ageestimate for the association, 10+10-7 Myr, agreeswith previous values cited in the literature. In the case of the twobrown dwarfs, we have derived an age of 15+15-10Myr, which also agree with our estimate for the whole group.Conclusions: .We compared our results with recent articles published onthe same subject using other techniques, and discuss the limits of theage-dating techniques.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample
We are obtaining spectra, spectral types, and basic physical parametersfor the nearly 3600 dwarf and giant stars earlier than M0 in theHipparcos catalog within 40 pc of the Sun. Here we report on resultsfor 1676 stars in the southern hemisphere observed at Cerro TololoInter-American Observatory and Steward Observatory. These resultsinclude new, precise, homogeneous spectral types, basic physicalparameters (including the effective temperature, surface gravity, andmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. We include notes on astrophysically interesting stars inthis sample, the metallicity distribution of the solar neighborhood, anda table of solar analogs. We also demonstrate that the bimodal nature ofthe distribution of the chromospheric activity parameterlogR'HK depends strongly on the metallicity, andwe explore the nature of the ``low-metallicity'' chromosphericallyactive K-type dwarfs.
| The Astrometric-Spectroscopic Binary System HIP 50796: An Overmassive Companion
We report spectroscopic observations of the star HIP 50796, previouslyconsidered (but later rejected) as a candidate member of the TW Hyaassociation. Our measurements reveal it to be a single-lined binary withan orbital period of 570 days and an eccentricity of e=0.61. Theastrometric signature of this orbit was previously detected by Hipparcosin the form of curvature in the proper-motion components, although theperiod was unknown at the time. By combining our radial velocitymeasurements with the Hipparcos intermediate data (abscissa residuals)we are able to derive the full three-dimensional orbit and determine thedynamical mass of the unseen companion, as well as a revisedtrigonometric parallax that accounts for the orbital motion. Given ourprimary mass estimate of 0.73 Msolar (mid-K dwarf), thecompanion mass is determined to be 0.89 Msolar, or ~20%larger than the primary. The likely explanation for the larger masswithout any apparent contribution to the light is that the companion isitself a closer binary composed of M dwarfs. The near-infrared excessand X-ray emission displayed by HIP 50796 support this. Our photometricmodeling of the excess leads to a lower limit to the mass ratio of theclose binary of q~0.8 and individual masses of 0.44-0.48 and 0.41-0.44Msolar. The new parallax (?=20.6+/-1.9 mas) issignificantly smaller than the original Hipparcos value and moreprecise.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| Radial Velocity Survey of Members and Candidate Members of the TW Hydrae Association
We report spectroscopic observations of stars belonging to the youngnearby group known as the TW Hydrae association, as well as of a numberof potential members of the association identified in kinematic andX-ray surveys. Multiple radial velocity measurements were obtained foreach object, several of which turn out to be multiple systems. Orbitalsolutions are presented for three double-lined binaries, onesingle-lined binary, and a double-lined triple system, all with shortperiods. Effective temperatures and projected rotational velocities arepresented for each visible object. None of the candidate members of theassociation in our sample are confirmed as a true member. The largefraction of close binaries among the candidate members has to do withtheir selection based on X-ray emission from ROSAT, which tends to favorthe inclusion of tidally locked systems that are active but notnecessarily young.Some of the observations reported here were obtained with the MultipleMirror Telescope, a joint facility of the Smithsonian Institution andthe University of Arizona.
| Additional TWA members?. Spectroscopic verification of kinematically selected TWA candidates
We present spectroscopic measurements of the 23 new candidate members ofthe TW Hydrae Association from \citet{MF}. Based on Hα and Li 6708Å strengths together with location on a color-magnitude diagramfor Hipparcos TWA candidates, we found only three possible new members(TYC 7760-0835-1, TYC 8238-1462-1, and TYC 8234-2856-1) in addition tothe already known member, TWA 19. This eliminated most of the candidatesmore distant than 100 pc. Three Tycho stars, almost certainly members ofthe Lower Centaurus Crux association, are the most distant members ofthe TWA. A claim of isotropic expansion of TWA has to be re-evaluatedbased on our new results. Generally, one cannot identify new members ofa diffuse nearby stellar group based solely on kinematic data. Toeliminate interlopers with similar kinematics, spectroscopicverification is essential.
| Internal kinematics of the TW Hya association of young stars
Thirty one probable kinematic members of the nearby TW Hya associationof young stars are selected from the RASSBSC/Tycho-2 sample of starsluminous in X-rays, detected by ROSAT. Eight of them have been listedalready as members of the association, and 23 are new candidates. Theassociation occupies a volume of some 106 pc3, thenearest member being at a distance of only 17.5 pc from the Sun. Thelower bound internal velocity dispersion is estimated at 0.8 kms-1, which is considerably larger than is expected in agravitationally bound open cluster. The total mass of the group isestimated at 31 solar masses. The centre of mass lies at 73 pc from usin the direction close to the position of the prototype star TW Hya. Arealistic kinematical model fitting both the observed trigonometricparallaxes and radial velocities involves a linear expansion of thegroup with the rate 0.12 km s-1 pc-1, whichdefines a dynamical age of 8.3 Myr, in good agreement with previous ageestimations for T Tauri members. Likewise the nearby Carina-Vela movinggroup of X-ray stars, the TW Hya association appears to be an outpost ofthe Gould Belt structure rather than an isolated open cluster.
| The Program to Link the HIPPARCOS Reference Frame to an Extragalactic Reference System Using the Fine Guidance Sensors of the Hubble Space Telescope
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2796H&db_key=AST
| HIPPARCOS distances of X-ray selected stars: implications on their nature as stellar population.
We present the parallaxes, measured by Hipparcos, for a sample of X-rayselected stars. The stars belong to the stellar sample of the EinsteinExtended Medium Sensitivity Survey. They are all at galactic latitude|b|>20deg, and are generally far away from known star formingregions. Several of these stars show lithium abundance and activitylevel typical of very young stars with ages comparable to that of thePleiades. We show that the majority of our sample stars are on the mainsequence, with only =~20% being giants. We do not find a significantpresence of pre-main sequence stars in our sample, notwithstanding thefact that some of our stars have a considerable lithium abundance,showing that the stars observed are most likely young and activemain-sequence objects.
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| Сазвежђа: | Секстант |
| Ректацензија: | 10h22m18.00s |
| Deклинација: | -10°32'15.5" |
| Apparent магнитуда: | 11.017 |
| Даљина: | 34.014 parsecs |
| Proper motion RA: | -74.9 |
| Proper motion Dec: | -25.3 |
| B-T magnitude: | 12.477 |
| V-T magnitude: | 11.138 |
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INHABITED SKY
